SUMMARY OF THE PRIOR ART
The perforating of cased and uncased well bores by a perforating gun carried on the bottom of a packer, which is run into the well on a tubing string, is an expedient widely utilized in the well drilling industry. After the perforating gun is discharged, it is generally desirable to drop the perforating gun assembly from the bottom of the tubing string if the guns get stuck or access through the tubing is desired for running tools such as pressure or temperature measuring units.
The release of a perforating gun from a tubular string is disclosed in U.S. Pat. No. 2,873,675 to LeBorg. In this patent, an annular powder charge is mounted in the upper portions of the perforating gun assembly and ignited by time delay fuse so that the annular charge does not detonate until after discharge of the perforating gun. The detonation of the annular charge effects the severing of the perforating gun from the tubular string by which the gun is supported. The utilization of the time delayed explosive charge to effect the gun release is obviously unreliable. In U.S. Pat. No. 3,706,344 to VANN, it is suggested that the perforating gun be severed from the tubing string by lowering a cutting tool into the well by wireline and effecting the severing after the discharge of the perforating gun. This is obviously a time consuming operation.
In recent years, a number of issued patents have proposed that the releasing of the perforating gun assembly from its supporting tubing string or packer be accomplished in response to a differential between tubing pressure and annulus pressure. These arrangements were again found to be unreliable due to the fact that the pressure differential between the tubing string bore and the annulus above a packer carried by the tubing string and conventionally set prior to the perforating operation, is a matter of conjecture. If the formation perforated by the discharge of the perforating gun has a high formation pressure, the resulting tubing string pressure may be sharply increased, resulting in a sufficient differential between the tubing string pressure and the annulus pressure to effect the dropping of the gun, but the operator at the surface will have no knowledge of the fact and will attempt to obtain the desired pressure differential to release the gun, even though it has already been dropped.
There is a need, therefore, for a method and apparatus for severing a perforating gun assembly from a supporting tubing string or packer which may be quickly and reliably accomplished from the well surface and which will immediately provide a reliable surface indication that that gun has been released and dropped to the bottom of the well.